PROJECT SUMMARY Within the United States, a growing proportion of the population is immunocompromised. Advances in transplantation techniques and cancer treatments, increased survival of these patients, and the development of novel agents to treat autoimmune diseases have contributed to the increase in this population. In the US, it is estimated that more than 340,000 people were living with a functioning transplanted solid organ (SOT) as of 2016, and over 56,000 new hematopoietic cell transplants and SOTs are done every year. Infection remains a major cause of morbidity and mortality for this highly susceptible population. Furthermore, these individuals are often affected by pathogens that rarely cause disease in healthy people. High-quality patient care and fostering collaborative, interdisciplinary research to increase our understanding of these infections, and drive development of new options for treatment and prevention, are vital priorities for improving outcomes among these patients. Conferences focused on infectious diseases specifically within immunocompromised patients are crucial for promoting timely discussions and efficient dissemination of ideas within the field, yet few such forums exist. To bridge this gap, the Fred Hutchinson Cancer Research Center's Infectious Disease Sciences Program launched in 2016 the Symposium on Infectious Diseases in the Immunocompromised Host. This event has attracted high-profile speakers and intense interest among those in the infectious diseases field, allowing us to expand the symposium to a two-day event with 176 attendees in the second year. Our 2019 symposium program will feature lectures by thought leaders in the field as well as early-career investigators, three keynote presentations, two interactive case discussions led by expert panels, and opportunities for trainees including presentations and two poster sessions. New topics addressed in our 2019 symposium program will include: Clostridium difficile, which disproportionately affects immunocompromised patients, and antimicrobial stewardship. Successful elements of our 2017 symposium will be maintained, including coverage of the following topics: Respiratory Virus Disease, Cytomegalovirus, Fungal Disease, Microbiome, and Immunotherapy. A distinct and innovative feature of this event is our focus on supporting attendance and participation by trainees, especially by women and individuals from underrepresented groups. We aim to continue this tradition by providing a generous number of trainee stipends and structuring our program to facilitate meaningful interactions between today's leaders in the field and the rising stars who will contribute to it for decades to come. We aim to foster productive discussions on infectious diseases in the immunocompromised host, driving progress on current issues in the field as well as promoting dissemination of knowledge that will improve clinical practice across the country. This conference strengthens the training pipeline by exposing early-career trainees to this enticing field, while supporting trainees (including those in our NIH-supported T32 training program) in honing their skills and gaining the visibility they need to launch independent careers.